The present invention relates more particularly to the field of double-walled pipes that include an insulating complex, in particular undersea pipes used for conveying hot or cold fluids, and still more particularly pipes constituted by sets of two coaxial pipes of the pipe-in-pipe (PIP) type that comprise an inner pipe and a coaxial outer pipe with an annular space between them, which space preferably contains thermally insulating material.
The present invention relates in particular to undersea pipes with reinforced insulation, of the kind installed on oil fields at great depths, and also to pipes that are suspended between the sea bottom and a surface ship anchored over said oil field.
More precisely, the present invention relates to a method and a device for inspecting welds at the ends of strings of double-walled pipes at the connections between each inner pipe and outer casing.
In most fields of industry, it is desirable to have high-performance insulation systems so as to maintain fluids being conveyed in pipework at constant temperature, so that transfers can be made possible between pieces of equipment over long distances, e.g. as long as several hundreds of meters or even several kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (approximately −165° C.), and off-shore oil fields extending over several tens of kilometers. Such oil fields are being developed in ever-increasing depths of water, which depths can be considerably greater than 3000 meters (m).
Numerous systems have been developed for achieving a high degree of thermal performance, and specific versions have been developed for providing better satisfaction for great depths, i.e. to be capable of withstanding the pressure at the bottom of the sea. The technologies presenting the best performance that have been developed for achieving this object are the so-called pipe-in-pipe technologies in which an inner pipe conveys the fluid and an outer pipe coaxial thereabout comes into contact with the ambient medium, i.e. the water. The annular space between the two pipes can be filled with an insulating material or it can be evacuated of all gas.
In pipes of this type, the annular space optionally filled with insulating material is generally at an absolute pressure that is less than atmospheric pressure, or else is evacuated of all gas, and to a first approximation it can be considered that the inner pipe provides the radial strength for withstanding the bursting pressure due to the internal fluid, while the outer casing withstands implosion due to the hydrostatic pressure at the depth of the bottom (ρ.g.h.) which is about 1 megapascal (MPa) per 100 m depth of water, i.e. MPa for a depth of 3000 m. The axial effect due to pressure, known as the bottom effect, acts on the circular section of the pipe, parallel to the axis of said pipe, and is distributed, to a first approximation, over both of the pipes (since they are connected together at their ends), prorata the respective sections of their material, generally constituted by steel.
For installations at great depths, undersea pipes and undersea sets of coaxial pipes are assembled on land to form unit pipe elements or strings each presenting a unit length lying in the range 20 m to 100 m depending on the size that can be handled by the laying system. They are then transported in this configuration out to sea on board a laying ship. During laying, the unit lengths of the various coaxial pipe assembly elements are connected to one another on board the ship, progressively while they are being laid at sea. It is thus important for the making of such connections to be an integral part of the process of mounting and assembling the pipe and of laying it on the sea bed, while delaying the process as little as possible, so it must be possible to make a connection both quickly and easily.
Patent FR 2 786 713 describes a method of making a connection at each of the ends of a string between an inner pipe and an outer pipe, the end of said outer pipe being set back from the end of said inner pipe, and being deformed by being “crooked” so that its inside diameter at its end is reduced substantially down to the outside diameter of said inner pipe, thus making it possible to make a circular weld connecting together the end of said outer pipe and said inner pipe.
Such internal welds cannot be inspected in conventional manner using X-rays or gamma rays since it is not possible to position a sensitive film circularly around the outside of the inner pipe so that it is centered on the welding bead and covers a certain width on either side of said welding, since the outer casing obstructs access to the zone situated between said outer casing and said inner pipe.
Devices are known for inspecting welding by an ultrasound probe. Such devices are generally operated manually by an operator and they are moved over and close to the weld, both axially in front-to-rear translation over the weld zone, and all around the circumference of the periphery of the pipe in said weld zone, as described in WO 01/71338.
Those devices are constituted by a piezoelectric emitter of ceramic material that emits an ultrasound wave for a short instance, which wave is then coupled into the mass of steel and the metal of the pipe and the weld, and becomes reflected on the opposite surface of the pipe. In general, said emitter also acts as a receiver sensor, with the wave corresponding to the received wave being delivered by electronic processor means to a computer.
Ultrasound probes are also known that comprise a series of emitter-receivers and that are referred to as “phased-array” or multibeam probes. The structure and the operation thereof is explained below, and in particular how to improve inspecting welds, it being possible for the various emitters to emit waves in a common plane perpendicular to the plane of the bottom face of the probe, but in various directions relative to said plane, thus making it possible to extend the geographical zone that is inspected when the probe is positioned in stationary manner, as explained below.
With that type of ultrasound device for inspecting welds, inspection can be performed correctly from the outside on one side only of the weld, towards the end of the string, with the other side of the weld in the space between the pipes remaining inaccessible because of the presence of said outer casing. However, it is possible to inspect the weld in part from the outer casing, except at the root of said weld, where the acoustic echo from the edge cannot be distinguished from a possible welding defect such as a crack initiation site.
Furthermore, by working from the inside, an operator has great difficulty in achieving accurate manual positioning of the ultrasound probe, not only because of the distance between the weld zone for inspection and the end of the pipe, but also because of the difficulty in accurately visualizing the exact position of the weld to be inspected, thus making it very difficult to show up critical welding defects. The problem is made worse when the diameter of the inner pipe is small. Furthermore, when the operator identifies a defect, the exact position of said defect is not known with great precision, specifically because the operation is performed manually.
While laying a PIP, the bending generated creates high levels of stress in the pipe, in particular in the connection zone between two successive lengths of PIP. That is why junction or coupling forgings of steel are often used that are assembled to the ends of the unit elements of sets of coaxial pipes for assembling together. The junction forging at the downstream end of a first unit element of a set of coaxial pipes that are not yet assembled together is connected to the junction forging at the upstream free end of a second set of coaxial pipes that have already been assembled together downstream.
Patents GB 2 161 565 and GB 2 191 842 describe a PIP and a method of assembling it, and also two embodiments of junction forgings or coupling forgings.
In the various embodiments, the junction forging generally includes two tubular branches comprising an outer branch and an inner branch defining an annular space between them, i.e. forming a fork with free cylindrical ends that are assembled to the substantially cylindrical ends respectively of the outer and inner pipes of said PIP type, assembly being performed by welding. The welding zones are then particularly sensitive to the phenomenon of fatigue, both during laying, and throughout the lifetime of the pipe, which is why it is important to be able to inspect the reliability of the welding.
Thus, the problem posed by the present invention is to facilitate and improve performing inspections on welds with the help of an ultrasound probe at the connection between an inner pipe and an outer pipe in a PIP pipe assembly, either when they are directly welded together, or else when they are welded together via forgings.